When you enter into any negotiation you must be prepared with a strategy. The strategy needs to include what you want as a business outcome. The first step is to have a plan on how the negotiation is going to proceed. As every situation involves people you must pay attention to their needs as well. When people are involved the situation becomes personal sooner or later. This part of your plan is to make sure you know as much as possible about the other person. Most people focus on the business side of the transaction. I believe at the end of the day all business is personal. Thus, finding out personal information moves the transaction away from just business and moves it toward what you might have in common. Those common interests help build a bond between you and the other individual. Then, the process becomes more like dealing with a friend.
I have found this approach helps create a setting where each side gains more of what they desire.
There will be times when trying to be friendly does not work. Sometimes the other side is looking to win at all costs. No matter what you do the other side is looking out only for themselves. Even in this scenario keep being nice because how you do things matters at all times. Your reputation transcends one deal.
Remember other people are always watching what you do or say.